1. Field of the Invention
This invention relates to a process and apparatus for reducing pollutant emissions including NO.sub.x, SO.sub.x, HCl, CO, total hydrocarbons (THC) and chlorinated hydrocarbons (CNC) in the flue gases derived from the combustion of combustible material, including municipal solid waste (MSW) and refuse derived fuel (RDF), in a high temperature furnace.
2. Description of the Prior Art
Most of the existing processes and apparatuses for combustion of combustible materials, and in particular, waste, such as municipal solid waste (MSW) or refuse derived fuel (RDF), include a combustion chamber equipped with a sloped or horizontal stoker grate that reciprocates or travels to move the combustible material from the combustible material inlet side of the combustion chamber to the ash removal side of the combustion chamber. A portion of the combustion air, generally equivalent to 1.0 to 1.3 of the combustible material stoichiometric equivalent, is supplied under the stoker grate. Such combustion air, typically called undergrate air, is distributed through the stoker grate to dry and combust the material present on the stoker grate. The combustible material is first dried on the drying portion or drying grate of the stoker grate, then combusted on the combustion portion or combustion grate of the stoker grate. Residual material from the combustion grate, primarily ash, is decarbonized or further combusted on the burnout portion or burnout grate of the stoker grate. The bottom ash is then removed through an ash pit. To assure carbon burnout, a high level of excess air above the stoichiometric level required for carbon burnout is maintained at the burnout grate. The combustion products from the stoker grate generally include carbon dioxide (CO.sub.2), water vapor (H.sub.2 O), nitrogen oxides (NO.sub.2), sulfur oxides (SO.sub.x), hydrogen chloride (HCl), carbon monoxide (CO), total hydrocarbons (THC) and chlorinated hydrocarbons (CNC). For environmental reasons, it is necessary to control the amount of emissions, and in particular, NO.sub.x, SO.sub.x, HCl, CO, THC and CHC, from this process released with the flue gases into the atmosphere. While CO and THC can be readily controlled by the addition of overfire air introduced above the stoker grate and mixed with products of combustion evolved from the stoker grate, reduction of NO.sub.x, SO.sub.x, HCl and CHC emissions requires a different approach. Indeed, the addition of overfire air which results in an excess air level downstream of the point of injection of the overfire air in the range of 60% to 100% of the stoichiometric requirement for complete combustion of the combustible material contributes to the formation of significant quantities of fuel NO.sub.x. Based on measurements by the inventors, typical mass burn operations result in about 30% of the total amount of NO.sub.x derived from the process generated on the stoker grate and about 70% at and downstream of the overfire air injection point.
In most cases, a boiler to recover heat generated by the combustion of the combustible material is an integral part of the combustion apparatus. In some cases, a portion of the flue gases from downstream of the boiler are recirculated back into the combustion chamber to reduce oxygen concentration and lower combustion temperatures, thereby inhibiting NO.sub.x formation. However, flue gas recirculation (FGR) generally results in higher concentrations of CO and THC within the flue gases, a significant disadvantage of using FGR as a technique for reducing NO.sub.x formation.
Known in-furnace processes for combined reduction of NO.sub.x, SO.sub.x, HCl, CO, THC and CHC require relatively long residence times for the products of combustion within the combustion chamber and generally provide no more than about 40%-50% SO.sub.x reduction and only up to 60% HCl reduction in the flue gases.
One known process for combined, but non-simultaneous, NO.sub.x, SO.sub.x, HCl, CO, THC and CHC reduction in flue gases from a boiler includes injecting hydrocarbon fuel into an area of the combustion chamber above the primary combustion zone and mixing the hydrocarbon fuel with the products of combustion from the primary combustion zone, forming a reducing zone which inhibits the formation of NO.sub.x due to the lack of oxygen and provides decomposition of fixed nitrogen species (FNS) such as NH.sub.3 and HCN within the zone. Overfire air is then injected into an oxidizing zone above the reducing zone to ensure complete combustion of combustibles in the combustion products exiting the reducing zone and entering the oxidizing zone. Finally, a sorbent, such as limestone or dolomite, is injected into still another zone above the oxidizing zone and mixed with the products of combustion from the oxidizing zone, thereby reducing the SO.sub.x and HCl content of the combustion products leaving this latter zone. Because of the non-simultaneous reduction of NO.sub.x, SO.sub.x, HCl, CO, THC and CHC in this process, longer residence times for combustion products within the various zones are required, requiring, in turn, generally larger combustion apparatuses.
Several techniques for reducing NO.sub.x emissions from combustion processes are taught in the prior art. U.S. Pat. No. 3,781,162 teaches an apparatus for mixing recirculated flue gases with combustion air prior to injection into a furnace to reduce the formation of NO.sub.x caused by the combustion of fuel.
U.S. Pat. No. 3,938,449 teaches the use of a rotary kiln for waste disposal in which the waste materials are combusted under stoichiometric conditions at temperatures below 2200.degree. F. to prevent the formation of NO.sub.x. The hot gases from the kiln are passed through a steam generator after which they are used to preheat and dehydrate the waste material prior to introduction into the kiln. As a final step, the gaseous output from the kiln is sent to a scrubber and then to evaporation ponds where solid material from the gases is deposited.
U.S. Pat. No. 4,336,469 teaches a method for operating a magnetohydrodynamic power plant in which fossil fuel is burned substoichiometrically in a combustor to produce a high temperature, fuel-rich product gas. A reducing agent, such as natural gas, is injected into the fuel-rich product gas as it passes from the combustor to a dwell chamber. The resulting mixture is retained in the dwell chamber for approximately one second, thereby permitting the reducing agent to decompose a portion of the NO.sub.x formed in the combustor. The fuel-rich product gas then passes through an afterburner wherein combustion is completed and any excess reducing agent is consumed.
U.S. Pat. No. 4,672,900 teaches a tangentially-fired furnace having injection ports for injecting secondary combustion air above the fireball in the combustion chamber to control NO.sub.x formation and eliminate flue gas swirl, thereby equalizing the temperature throughout the flue gases as they exit the combustion chamber and enter the economizer section of the furnace.
Related U.S. Pat. Nos. 4,013,399, 4,050,877 and 3,955,909 teach the reduction of gaseous pollutants in flue gases using two-stage combustion. Fuel is burned in a combustion chamber under substoichiometric conditions and at temperatures below that at which significant NO.sub.x would be produced. The combustion gases are passed through a secondary combustion zone into which additional air has been injected through a plurality of foraminous tubes for completion of the combustion process. The temperature of the secondary zone is also maintained below the temperature at which significant amounts of NO.sub.x would be formed.
U.S. Pat. No. 4,589,353 teaches a furnace for burning wood chips or other cellulose fuel in which the cross-sectional area of the furnace increases with increasing furnace height to reduce the velocity of upwardly flowing combustion products. As a result, any partially combusted particles initially picked up by the upwardly flowing combustion gases reach a height where the gas velocity equals the particle terminal velocity, at which point the remain suspended until they have been further combusted and reduced in size to be carried out of the furnace by lower velocity gases. Air to support the combustion is introduced through openings beneath grates supporting the combustible material. Secondary air is injected into the furnace above the grates creating an oxidizing secondary combustion zone.
U.S. Pat. No. 4,538,529 teaches a nozzle box for blowing secondary air streams at high velocity into a stream of flue gas emanating from the combustion of garbage on a combustion grate creating a secondary combustion zone in which volatile components (fumes) generated from the combustion of the garbage are completely combusted.
U.S. Pat. Nos. 4,624,192, 4,646,661 and 4,628,833 teach the use of vitiated air in stoker-type furnaces.
Finally, U.S. Pat. No. 4,779,545 discloses a method and apparatus for reducing NO.sub.x emissions from furnace flue gases by injecting pulses of natural gas or other fluid fuel which has little or no fixed nitrogen into the upper portion of the furnace where it mixes with the NO.sub.x -laden flue gases from the combustion of coal in the lower portion of the furnace, forming ammonia-like compounds and nitrogen gas. The ammonia-like compounds react with additional amounts of NO.sub.x in the flue gas to form nitrogen gas, water vapor and carbon dioxide, resulting in the reduction of NO.sub.x in the flue gases.
Of the prior art discussed hereinabove, none discloses or suggests a process for the simultaneous reduction of NO.sub.x, SO.sub.x, HCl, CO, THC and CHC in the flue gases from the combustion of combustible material, such as municipal solid waste and refuse derived fuel, in a combustion chamber.